A solder resist (or protective film) is a protective film for a circuit pattern that is formed on the outermost surface of a print substrate, and stringent precision requirements have arisen from current trends of high-precision printed circuit boards.
Recently, a photosensitive protective film (photoimageable coverlay) for a circuit board has been used in a photolithography process using a photosensitive resin composition in order to improve a fine circuit pattern and precise positioning.
In general, the photosensitive protective film for a circuit board is prepared by thermally compressing a liquid or film type of photosensitive resin composition on a circuit of a copper clad laminate (CCL), exposing it to UV along the pattern, developing it with a developing agent, washing it, drying it, and performing thermal curing. Then, fine holes required to connect a circuit can be formed precisely at desired positions.
The conventional solder resist for a circuit board was obtained from a photosensitive resin composition prepared by adding an acrylate to an epoxy resin.
However, such a conventional photosensitive resin composition can be discolored or delaminated from the circuit in a soldering process due to poor flame retardancy or poor heat resistance after curing. Additionally, the resin composition is not suitable for a photosensitive protective film of the circuit board because it cannot be applied to regions requiring repeated foldings due to its insufficient flexibility and bending resistance.
In order to improve flame retardancy or heat resistance of the conventional photosensitive resin composition, bromine-containing aromatic compounds or antimony compounds have been further added. However, these added substances create dioxins when burned or they in themselves are harmful to the human body, and thus, there is a difficulty in meeting current stringent regulatory requirements.
Accordingly, there has been a demand for a photosensitive resin composition that is excellent in heat resistance, flame retardancy, bending property, and insulation reliability, and that is environmentally friendly and harmless to the human body. In recent years, polyimides or a precursor thereof has been actively used.
In detail, since polyimides and precursors thereof have excellent durability, heat resistance, flame retardancy, and mechanical and electronic properties, they are actively used as a base film of a printed circuit board, as well as a cover film for a highly integrated semiconductor device or a highly integrated multilayered wiring board. Thus, a polyimide photosensitive resin has been highly demanded because the problems of the conventional photosensitive protective films can be overcome or solved.
In spite of the demand, there are several obstacles in using a polyimide as a material for the photosensitive circuit protective film.
Specifically, in application of the polyimide photosensitive resin, a polyamic acid, which is a precursor of a polyimide, is used so as to increase the formability. However, polyimidization of the polyamic acid requires a high temperature of 350° C. or higher.
Therefore, if it is polyimidized through a heat curing process while it is applied onto the circuit pattern in a polyamic acid form or attached in a film form, there are problems that a copper circuit that is vulnerable to heat may be undesirably oxidized and deteriorated.
Moreover, to achieve appropriate properties, a polyimide is required to have a predetermined molecular weight. If the molecular weight is increased, compatibility with other substances is reduced, it is hard to realize the developing property with respect to the weak alkaline aqueous solution, and the space-filling capability is reduced due to a high modulus. Thus, there is a limit in development of a circuit protective film that is made of a polyimide as the photosensitive protective film.